MHC Class-II Expression Reveals Differential Regulation of BLIMP-1 Target Genes.

BLIMP-1 initiates plasma cell differentiation by inhibiting the expression of a limited set of transcription factor genes. BLIMP-1 mediates repression of its targets by promoter occupancy and recruitment of the Groucho corepressor, histone deacetylase and the G9a lysine methyltransferase. The relative contribution of these mechanisms to the stable silencing of BLIMP-1 target genes in plasma cells is not known. Epigenetic changes initiated by BLIMP-1 could be maintained by independent factors or alternatively may be dependent on continuous occupancy of the promoter by BLIMP-1. The aim of this study was to investigate the mechanisms operating at two well-defined BLIMP-1 target genes, MHC2TA and PAX-5. Repression of PAX-5 is essential for extinction of the B-cell phenotype, while repression of MHC2TA mediates the loss of MHC class-II expression characteristic of plasma cell differentiation. PAX-5 is silenced in virtually all cases of myeloma, whereas MHC class-II continues to be expressed in a subset of primary myeloma cells and many myeloma cell lines. Using chromatin immunoprecipitation we demonstrate that BLIMP-1 constitutively binds MHC2TA promoter-3 and is present equally in MHC2TA expressing and non-expressing myeloma cell lines. In expressing cells MHC2TA promoter-3 is associated with acetylation of histone H3 lysine-9, a marker of open chromatin, whereas histone H3 lysine-9 is neither acetylated nor methylated in non-expressing cells. In contrast the PAX-5 promoter is associated with trimethylation of histone H3 lysine-9, a marker of repressed heterochromatin, in all the cell lines examined, but constitutive BLIMP-1 occupancy is not detectable. Although BLIMP-1 does not silence MHC2TA expression in the MHC class-II positive U266 cell line, it is mediating transcriptional repression, since siRNA knockdown of BLIMP-1 is associated with elevated MHC2TA mRNA and MHC class-II surface expression. In contrast PAX-5 mRNA and its target CD19 continue to be repressed. MHC class-II re-expression was associated with loss of CD138 and the entry of the majority of cells into apoptosis. Our data demonstrate that MHC class-II expression in myeloma cell lines is associated with defective silencing of MHC2TA despite BLIMP-1 occupancy, and reveal differences in the contribution of epigenetic modifications and promoter occupancy in the maintenance of target gene silencing by BLIMP-1. Loss of BLIMP-1 results in a partial reversion of plasma cell phenotype that is associated with induction of apoptosis. As continual high levels of BLIMP-1 expression appear to be critical for plasma cell survival, suppression of BLIMP-1 represents a potential therapeutic pathway.